This invention relates generally to the etching of polyimide material, and more particularly to a method of etching polyimide to provide a surface surrounding the etched areas which has topography suitable for reliable interconnection and metal adherence.
In many different processes it is necessary to provide a polyimide film with openings therein. For example, in the packaging of semiconductor chips polyimide films are often coated on substrates. In certain of these uses it is necessary to form openings in the polyimide coating to allow for electrical connections to be made between various levels of metallurgy. These openings, commonly referred to as vias, are conventionally formed by etching the polyimide with a strong basic reagent in the areas where polyimide is to be removed to form the vias. However, it is necessary to perform this etching with the polyimide in an uncured or semi-cured state since fully cured polyimide will not react with strong bases. Therefore the conventional practice has been to apply a polyimide precursor, and then remove the solvent and partially cure applied material to form a polyimide. This partially cured film is then etched with a strong base to form the desired vias after which etching the film is fully cured. After the film is fully cured the necessary metal is deposited in the vias to afford contact with the underlying metal.
It has been found however, that there is a problem in obtaining proper continuity of the metal, through the vias. Upon investigation it has been discovered that when the partially cured polyimide is etched, there is a thin surface layer of polyimide material left at the walls of the via which has absorbed some of the etchant, but has not been removed. This absorbed etchant alters the ability of the polyimide to further cross-link such that it will not fully cure, but rather remain in a gel like consistency even after the curing cycle is completed which would normally fully cure the polyimide and which does fully cure the remainder of the polyimide film.